This invention relates generally to anesthesia machines, and, more particularly, to pressure relief valves, usually referred to as pop-off valves, for the evacuation of excess gas from a patient breathing system used in conjunction with a gas anesthesia machine. In a gas anesthesia machine, an anesthetic gas, such as nitrous oxide (N.sub.2 O), is mixed with oxygen and, in some applications, passed through a vaporizer before emerging at a common outlet from the machine.
The common outlet is connectable directly to a patient breathing system, which is typically a closed system in which a patient undergoing anesthesia breathes in both recirculated gas, and gas supplied from the common outlet. Recirculated gas is obtained by processing exhaled gas through a carbon dioxide absorber and merging this flow with that from the common outlet. Check valves in the breathing system prevent the patient from exhaling directly back toward the common outlet, and from inhaling from the inlet side of the carbon dioxide absorber. Also included in the circle breathing system is a breathing bag, which fills with gas when the patient exhales and empties again when the patient inhales, some of the gas in the bag being recirculated through the carbon dioxide absorber and back to the patient.
Alternatively, if the patient is incapable of breathing, the breathing bag may be replaced by a gas-driven ventilator bellows, which fills during exhalation of gas from the patient's lungs and is emptied when drive gas is applied to the outside of the bellows during an inhalation phase of the breathing cycle. Application of the drive gas is controlled in response to sensed pressure, flow rate, or both, in the tube through which the patient breathes.
In one sense, the patient breathing system is a closed system, since some of the gas exhaled by the patient is processed by the absorber and recycled to the patient. However, since fresh anesthesia gas and oxygen may be continually supplied through the common outlet, the system is not a completely closed one, and some means must be provided to evacuate excess gas from the breathing system, to compensate for the gas drawn from the common outlet. Regardless of whether the breathing bag or the ventilator is used, a relief valve is provided to vent the excess gas at some stage during each breathing cycle. The relief valve used in conjunction with the breathing bag is referred to as a pop-off valve, and it is with this valve that the present invention is concerned. The vented gas is not allowed to escape directly to atmosphere, but is passed through an evacuator interface unit to a vacuum line. However, the gas evacuator interface forms no part of the present invention.
The pop-off valve typically comprises a relatively lightweight valve closure element which is spring biased against a valve seat. When a predetermined pressure is applied beneath the valve closure, it is lifted from the seat and gas is vented through an outlet port. The pressure threshold at which the valve opens is adjustable by varying the biasing force on the valve, and may be as low as a few centimeters of water to 10 or more centimeters of water. The valve has to be able to handle flow rates up to approximately 15 liters per minute, and ideally the valve opening pressure should not increase unduly as the flow rate increases.
In the past, pop-off valves have been subject to a number of significant disadvantages. First, pop-off valves have been subject to a vibration condition known as "honking," wherein the valve closure oscillates with respect to the valve seat. The resultant vibration is transmitted throughout the breathing system, and can induce vibration in other valves in the system. Dampening chambes on the downstream side of the pop-off valve have been used to minimize transmission of "honking" noise, but have not been completely successful. In addition, some prior art valves have not been able to operate at extremely low threshold pressures, below a few centimeters of water, and have a pressure-flow characteristic wherein the threshold pressure increases substantially as the flow rate is increased.
Accordingly, there has been a significant need for a pop-off valve that overcomes these disadvantages of prior art valves. The present invention satisfies this need.